Red blood cell (RBC) transfusion remains a very common therapeutic in intensive care units, but is associated with unintended consequences in the critically ill population. While the lungs are a target organ, human lung injury is seldom caused by any single event, and RBC transfusion alone is generally not sufficient to induce injury but requires a host with increased susceptibility. Our ongoing interest in RBC transfusion and its potential role in modifying inflammatory responses have led us to focus upon the role of transfused oxidized, damaged RBC (oxRBC) and micro-particles bearing surface phosphatidylserine (PS) in altering the mononuclear phagocyte activation status and the role of host factors that protect from persistent lung inflammation and injury. We hypothesize that an endogenous countering mechanism exists whereby macrophage (M?) engulfment of transfused oxRBC or PS+ micro-particles contained within standard RBC units results in suppression of macrophage activation through the release of the anti-inflammatory signal IL-10. This de-activating signal resulting from the ingestion of apoptotic bodies is required to curtail inflammation and promote the resolution phase of injury. Improper deactivation, through defective IL-10 signaling following ingestion of apoptotic bodies may increase the susceptibility of an individual to the risks of RBC transfusion. We have identified a novel murine model of the susceptible host and propose that mice deficient in thrombospondin-1 (TSP1), a multi-functional adhesive glycoprotein involved in phagocyte recognition of PS+ apoptotic bodies through the M? scavenging receptor CD36, show defective IL-10 responses following LPS- induced lung inflammation and fail to resolve injury in addition to defective IL-10 signaling in th liver following transfusion of oxRBC. Based upon our findings, we propose an overall hypothesis that TSP1 functions as an extracellular, bridging molecule that mediates CD36-dependent IL-10 production, contributing to M? deactivation necessary for proper resolution of lung inflammation following transfusion. When disrupted, the host is left vulnerable to the harmful effects of RBC transfusion and shows persistence of inflammation through impaired M? deactivation. Utilizing human monocyte derived M? (HMDM), murine M?, tsp1-/-, and cd36-/- mice, we will determine whether (1) TSP1-mediated IL-10 production by M? following engagement of RBC micro-particles or oxRBC requires CD36; (2) transfusion of stored RBC impairs resolution of lung inflammation induced by either LPS or bacterial pathogen in tsp1-/- mice through CD36-dependent IL-10 production; and (3) reconstitution of IL-10 hastens resolution of lung inflammation in tsp1-/- mice and that immune-modulatory properties of RBC micro-particles can be harnessed to reprogram mononuclear phagocytes with intact TSP- 1/CD36 axis toward resolution. Completion of these studies will identify a highly novel pathway underlying the immune-modulatory aspect of RBC transfusion and its contribution to lung inflammation and injury in the susceptible host.